Provides read-only access to the render state.
The render state is represented by the render parameters,
portlet mode and window state.

During render or resource request processing, the render state is constant and
can be accessed through methods provided by this interface.

During portlet action or event request ptocessing, the render state can be modified for
subsequent render phase processing.
During the portlet render or resource phases, URLs containing modified render state
can be created.
When a portlet URL containing modified render state is triggered, the corresponding portlet
phase is executed with the modified render state.